Orthogonal frequency-division multiplexing (OFDM) and Nyquist-WDM (wavelength-division multiplexing) are regarded as likely technologies for improving frequency utilization efficiency. In optical transmission systems where transmission rates of 100 Gbps or greater are demanded, the concept of a superchannel is adopted, in which OFDM and Nyquist-WDM are used to multiplex a plurality of subchannels which are then treated as a single signal.
In addition, there exists technology that improves the edge transmittance of an optical waveform transmitted through the band pass filter of an optical add-drop multiplexer (OADM), such technology being implemented in order to address edge deterioration in optical waveforms transmitted through such a filter. For more information, see S. Chandrasekhar and Xiang Liu, OFC2011 OMU5. S. Chandrasekhar and Xiang Liu, OFC2011 OMR5 is an example of related art.
FIG. 31 illustrates an example of signal quality variation occurring along a lightpath. When an optical transmission apparatus 201 in an optical transmission system 200 sends a superchannel signal on a lightpath 202, wavelength-dependent gain or loss in optical components such as optical amplifiers and optical filters produce signal quality variation among the subchannel signals in the superchannel signal.
The optical transmission apparatus 201 also has a built-in wavelength selective switch (WSS) 211. The WSS 211 adjusts the gain or optical power level of each subchannel signal in a superchannel signal received over the lightpath 202, and sends the adjusted superchannel signal over the lightpath 202.
FIG. 32 illustrates an example of narrowing being produced in the edge band portion of a superchannel signal. As a superchannel signal is repeatedly added and dropped on the lightpath 202, the spectral widths become narrower for subchannel signals in the edge band portion on both sides due to the transmittance properties of the WSS 211, causing worsened optical transmission quality.